Field of the Invention
The present invention relates to a driving technique for a light-emitting diode.
Description of the Related Art
As a backlight for a liquid crystal display (LCD) panel, white light-emitting diodes (which will be simply referred to as “LEDs” hereafter) having favorable properties from the perspective of a long operating life, low power consumption, and a wide color range are employed.
FIG. 1 is a diagram showing a backlight illumination apparatus 2r investigated by the present inventors. FIG. 1 shows an LCD panel 102 in addition to the illumination apparatus 2r. The illumination apparatus 2r is configured as a direct-type illumination apparatus. The illumination apparatus 2r includes: LED bars (which will also be referred to as the “LED strings”) 10_1 through 10_4 respectively provided to multiple channels CH1 through CH4 on the back face of the LCD panel 102; and a multi-channel driving circuit 20 that drives the LED bars 10_1 through 10_4.
Each LED bar 10 includes multiple LEDs connected in series. The LCD panel 102 has multiple regions 104_1 through 104_4, which are vertically divided along a first direction. The multiple LED bars 10_1 through 10_4 are provided to the multiple regions 104_1 through 104_4, respectively. Specifically, the LED bar 10_i of the i-th channel CHi is arranged on the back face of the LCD panel 102 such that it is assigned to the corresponding i-th region 104_i.
The driving circuit 20 includes multiple current drivers 22_1 through 22_4, a DC/DC converter 24, and a control circuit 26. The current drivers 22_1 through 22_4 are each provided to the corresponding channel, and are directly connected to the corresponding LED bars 10_1 through 10_4, respectively. The luminance of the LED bar 10 of each channel is controlled according to a driving current ILED generated by the corresponding current driver 22.
A DC/DC converter 24 supplies a driving voltage VOUT across both ends of each channel of the LED bar 10 and the current driver 22. The control circuit 26 controls the DC/DC converter 24 so as to stabilize the driving voltage VOUT to a level that allows the LED bar 10 of each channel to emit light with desired luminance Furthermore, the control circuit 26 controls a driving current ILED generated by the current drivers 22_1 through 22_4.
There are two known methods for controlling the luminance of a backlight LED, i.e., a current dimming (analog dimming) control method and a PWM dimming (pulse dimming) control method. With the current dimming control method, the current value of the driving current ILED that flows through the LED bar 10 is controlled. With the PWM dimming control method, the driving current ILED is switched on and off with a frequency from several dozen to several hundred Hz so as to adjust the time ratio (duty ratio) between the on period in which the flow of the driving current ILED is supplied and the off period in which the flow of the driving current ILED is suspended. Such an arrangement is capable of adjusting the effective luminance of each LED bar 10.
FIG. 2 is a circuit diagram showing an illumination apparatus 2r according to a conventional technique. A control circuit 26 (200r) according to such a conventional technique includes an error amplifier 202, a pulse width modulator 204, a driver 206, and a dimming controller 210. The cathode voltages (which will also be referred to as the “detection voltages”) VLED1 through VLEDN of multiple respective LED bars 10_N are fed back to the control circuit 200r. The cathode voltage VLED of each channel corresponds to the voltage across both ends of the corresponding current driver 22.
The error amplifier 202 amplifies the difference between the lowest voltage from among the multiple cathode voltages VLED1 through VLEDN and a predetermined reference voltage VREF. The pulse width modulator 204 generates a pulse signal SPWM having a duty ratio that corresponds to an error voltage VERR received from the error amplifier 202. The driver 206 switches on and off the DC/DC converter 24 according to the pulse signal SPWM.
The dimming controller 210 instructs the current driver 22 of each channel to generate the driving current ILED with a current value that is changed according to the target luminance set for the corresponding LED bar 10 (analog dimming control method). Furthermore, the dimming controller 210 instructs the current driver 22 of each channel to generate the driving current ILED with a time ratio that is changed according to the target luminance set for the corresponding LED bar 10 (PWM dimming control method).
In a case in which the PWM dimming control operation is performed, when a given channel is in the off state, that channel is excluded from the candidates used by the error amplifier 202 to perform the feedback control operation. That is to say, the error amplifier 202 uses only the channels in the on state as the candidates to perform the feedback control operation. With such an arrangement, the cathode voltage VLED1 of the i-th channel is represented by the following Expression.VLED1=VOUT−VFi 
Here, VFi represents the forward voltage VFi of the LED bar 10_i of the i-th channel. That is to say, with the illumination apparatus 2r shown in FIG. 2, the channel that provides the lowest cathode voltage VLED, i.e., the channel of the LED bar 10 with the highest voltage drop (forward voltage VF), is used to perform the feedback control operation. Thus, the output voltage VOUT of the DC/DC converter 24 is stabilized to a value represented by the following Expression.VOUT=VREF+VF_MAX.
Here, VF_MAX represents the highest forward voltage VF from among the on-state channels.
In order to provide an image with a wide dynamic range of luminance, an advanced display apparatus supports an individual area dimming control function. In the individual area dimming control operation, when a given LED bar 10 is to provide the corresponding region 104 with a high luminance, the LED bar 10 emits light with a high on-time ratio (high duty ratio) according to an image to be displayed on the LCD panel 102. Conversely, when a given LED bar 10 is to provide the corresponding region 104 with a low luminance, the LED bar 10 emits light with a low on-time ratio (low duty ratio). FIG. 3 is an operation waveform diagram showing the operation of the control circuit 200r in a case in which it performs such an individual area dimming control operation.
The current driver 22 of each channel switches on and off with a duty ratio that is dynamically and adaptively changed according to an image to be displayed on the LCD panel 102. With such an arrangement, in some cases, there is a large difference in the forward voltage VF among the LED bars 10 due to variation in the elements of the multiple LEDs that form each LED bar 10. Description will be made regarding an example including four channels of LED bars 10 assuming that the relation VF1>VF2>VF3>VF4 holds true. In FIG. 3, the PWMi signal (i=1, 2, 3, 4) represents the on state (high level) and the off state (low level) for the i-th channel.
As shown in FIG. 3, with the illumination apparatus 2r shown in FIG. 2, the DC/DC converter 24 outputs the output voltage VOUT, which changes with time according to the PWM signal. As the difference between the forward voltages VF1 through VF4 of the multiple respective channels becomes larger, the range in which the output voltage VOUT changes becomes larger.
When the channel used to perform the feedback control operation is switched from a channel at which the forward voltage VF is low to another channel at which the forward voltage VF is high, the output voltage VOUT is raised. However, in a case in which the rising rate of the output voltage VOUT is insufficient, the voltage VLED across both ends of the current driver 22 is lower than that required for the channel at which the forward voltage VF is large. In this state, such a current driver 22 is not able to supply a sufficient driving current ILED, leading to a reduction in the luminance provided by the LED bar 10 of this channel. Such a reduction in the luminance is recognized by viewers as screen flicker, which is a problem.
In order to solve such a problem, the control circuit 200r is required to have an improved response speed. In order to support such a high response speed, there is a need to raise the switching frequency for the DC/DC converter 24. However, this leads to a problem of increased power consumption and increased heat generation. Furthermore, in a case in which the feedback loop has a wide bandwidth, this leads to degraded phase characteristics, resulting in a reduction in the stability of the feedback loop. Moreover, in a case in which the output voltage VOUT is changed at a high rate, such an arrangement leads to a problem of noise generation from ceramic capacitors, coils, or the like.
Such problems are not restricted to such a backlight illumination apparatus 2r, but can occur in various kinds of illumination apparatuses for various kinds of usages.